Abstract
The spindle checkpoint senses unattached or improperly attached kinetochores during mitosis, inhibits the anaphase-promoting complex or cyclosome (APC/C), and delays anaphase onset to prevent aneuploidy. The mitotic checkpoint complex (MCC) consisting of BubR1, Bub3, Mad2, and Cdc20 is a critical APC/C-inhibitory checkpoint complex in human cells. At the metaphase-anaphase transition, the spindle checkpoint turns off, and MCC disassembles to allow anaphase onset. The molecular mechanisms of checkpoint inactivation are poorly understood. A major unresolved issue is the role of Cdc20 autoubiquitination in this process. Although Cdc20 autoubiquitination can promote Mad2 dissociation from Cdc20, a nonubiquitinatable Cdc20 mutant still dissociates from Mad2 during checkpoint inactivation. Here, we show that depletion of p31(comet) delays Mad2 dissociation from Cdc20 mutants that cannot undergo autoubiquitination. Thus both p31(comet) and ubiquitination of Cdc20 are critical mechanisms of checkpoint inactivation. They act redundantly to promote Mad2 dissociation from Cdc20.